1. Field of the Invention
The present invention relates to a paper conveying device for conveying paper such as a document, and more particularly, to an automatic paper conveying device mounted on a copying machine or the like for automatically setting paper such as a document in a predetermined position.
2. Description of the Related Art
An automatic document conveying device mounted on a copying machine, a facsimile or the like for conveying a document which is one type of paper so as to automatically set the document in a predetermined position has been known.
Many of conventional automatic document conveying devices include a mechanism for conveying a document by a conveying belt. In this type of automatic document conveying device, control is so carried out that when the document is set, the front end or the rear end of the document is stopped along a predetermined reference line.
One of methods of control conventionally carried out so as to exactly stop the end of a document along a reference line is a method of overrunning once the end of a document by a predetermined amount from the position where the document is set and then, driving a conveying belt in the reverse direction to switch the document back so that the end of the document collides with a projection provided in the position of the reference line. In such a method of switching the document back, control for overrunning the document and switching the document back has been conventionally managed by time.
The method of switching the document back so that the end of the document collides with the projection provided in the position of the reference line has the advantage that the document can be exactly stopped in the position of the reference line, while having the disadvantage in that the stress is applied to the document at the time of the collision and consequently the document is liable to be, for example, wrinkled or folded. Particularly in the method of managing the amount of overrun and the amount of switchback of the document by time in the prior art, sufficient time for management must be set and a sufficient amount for switchback is set. Accordingly, a force is exerted on the document after the end of the document collides with the projection, to increase the stress applied to the document.
Since sufficient time for management is set, the document is overrun and switched back by a distance greater than necessary in many cases. Therefore, the system also has the disadvantages in that it takes long to set the document and it also takes long to replace the document with another document.
In order to eliminate the disadvantages in the prior art, the applicant of the present application has previously proposed an improved paper conveying device (see U.S. patent application Ser. No. 08/077,010).
The paper conveying device previously proposed is so constructed that control of a motor for driving a conveying mechanism is improved to overrun paper by the minimum distance required and switch the paper back exactly by the amount of overrun. Therefore, the paper conveying device is superior in that the stress applied to the paper can be reduced and consequently the paper is hardly damaged, and time required until the paper is stopped in a predetermined position may be short.
In putting the above described paper conveying device in the prior application to practical use, however, the following becomes clear. Specifically, if the paper conveying speed is lower than the standard speed in a case where the paper is overrun by coasting, the coasting distance of the paper does not reach a distance required for control. Specifically, some problems are found out. For example, if the conveying speed is low, the coasting distance becomes short, so that a part of the end of the paper may not, in some cases, overrun the position of a reference line.